Abstract Autosomal-dominant (ADPKD) is the most common life-threatening genetic disease. We have recently found that polycystin-1 (PC1), the protein affected in ADPKD, functions in the regulation of STAT6 activity by ciliary mechanotransduction in renal epithelial cells. PC1 undergoes flow-dependent proteolytic cleavage which releases its cytoplasmic tail from the membrane, followed by nuclear translocation. The PC1 tail binds to STAT6 and the transcriptional co-activator P100, and stimulates STAT6-dependent gene expression. STAT6 translocates from primary cilia to nuclei upon cessation of fluid flow. The nuclear PC1 tail is highly expressed in cyst-lining epithelial cells in ADPKD. Expression of the PC1 tail stimulates proliferation in MDCK cells and results in renal cyst-formation in zebrafish embryos. Furthermore, MDCK cells respond to interleukin-4 (IL4) and IL13 similar to immune cells in that they activate STAT6, and up-regulate the expression if IL4/13 receptor chains. These results strongly suggest that PC1 functions to silence STAT6 activity in the normal healthy kidney. Cessation of lumenal fluid flow, e.g. due to renal injury, triggers PC1 cleavage, STAT6 activation and a proliferative response. We hypothesize that lack of functional PC1 in ADPKD leads to constitutive STAT6 activity and an aberrant proliferative "repair" response leading to cyst growth. Our preliminary results show that a clinically approved drug, known to inhibit STAT6, strongly inhibits renal growth and preserves renal function in a polycystic mouse model. We now propose to study in detail the mechanism of the regulation of STAT6 activity by PC1 and to test our hypotheses regarding the role of this novel signaling pathway in renal injury repair and ADPKD. In Aim 1, we will characterize the regulation of STAT6 in MDCK cells expressing a STAT6-responsive GFP reporter. The effects of the state of differentiation, PC1 tail expression, apical fluid flow and IL4/13 will be investigated. We will test whether STAT6 is activated in a scratch-wounding model. We will investigate the expression and localization/secretion of IL4/13 and IL4/13 receptor chains in response to STAT6 activation. Analysis of ADPKD tissue and polycystic mouse models will reveal whether IL4/13 receptors are up-regulated in cysts and whether they secrete IL4/13 into the lumen. Finally, we will test our hypothesis that the IL4/13/STAT6/PC1 pathway is activated in a mouse model of renal ischemia/reperfusion injury. In Aim 2, we will test whether crossing of STAT6 null mice with polycystic mouse models will result in suppression of renal cystic disease. Furthermore, we will investigate the amelioration of renal cystic disease by treatment with the STAT6 inhibitor and delineate the mechanism of action. In Aim 3, we will generate a transgenic mouse line over-expressing the soluble PC1 tail in a kidney-specific and doxycycline-inducible manner. Based on our results in MDCK cells and zebrafish, we anticipate that these animals will develop renal cystic disease and will mimic human ADPKD most closely mechanistically.